The invention relates to a method and device for detecting the occurrence of a flash in an apparatus comprising a cathode ray tube (CRT), such as for instance a television set, a monitor, etc.
In such apparatus, an image is projected on a screen by means of an electron beam, which scans the screen in a horizontal direction and in a vertical direction. For generating the electron beam, a high voltage needs to be generated; for scanning the screen in the horizontal direction, a horizontal deflection circuit is necessary. In principle, these two circuits could be separate circuits; however, the present invention relates more particularly to an apparatus of the above-described type that comprises a combined high voltage and horizontal deflection circuit.
In practice, although rarely, it may happen that a fail situation occurs in the high voltage circuit involving a flash from one part carrying a high voltage to another part. The most probable cause of a flash is a particle coming loose from the inner cone coating and depositing on a high voltage part, locally increasing the field strength. The high currents involved in such flash usually cause evaporation of such particle and, if no real damage has been caused by such high currents, the apparatus may continue its functioning after the flash. However, the flash may also cause a controller of the apparatus to enter an undesired or undefined state. As a result, the apparatus does not operate as expected by the user who, believing the apparatus is damaged, may incur expenses for reparation. However, said controller could, in principle, be taken out of such undesired or undefined state by a simple reset action or the like, thus keeping the disturbance for the user at a minimum.
It is, inter alia, an object of the invention to provide a method and device for detecting in a reliable way whether a flash has occurred. Then, an alarm signal that is indicative of the occurrence of a flash may be generated. Also, an action signal may be generated, which can be used as a control signal for said controller, for instance to perform a reset. To this end, the invention provides a flash detection as defined in the independent claims. Advantageous embodiments are defined in the dependent claims.
The present invention is based primarily on the recognition of the fact that, in an apparatus comprising a combined high voltage and horizontal deflection circuit, the flash occurring in the high voltage part of the circuit will also affect the operation of the horizontal deflection part of this circuit. Thus, based on this recognition, the present invention provides a method and device for monitoring output signals of the horizontal deflection transistor, and for generating said alarm signal or action signal, respectively, on the basis thereof.
In a first embodiment, the shape of the fly-back pulse is monitored; if, after the end of the normal fly-back pulse, the fly-back voltage increases significantly within a predefined time window, said alarm signal or action signal, respectively, is generated.
In a second embodiment, the storage time of the line deflection transistor is measured for consecutive lines, and the consecutive measured values are compared to each other; if a significant deviation is determined, said alarm signal or action signal, respectively, is generated.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.